Various processes and catalysts exist for the copolymerization of alpha-olefins. For example, U.S. Pat. No. 3,113,115 (Ziegler et al) discloses that a plastomeric copolymer of ethylene and propylene may be prepared using a catalyst system comprising diethyl aluminum chloride and titanium tetrachloride while U.S. Pat. No. 3,300,459 (Natta et al) discloses that an elastomeric copolymer of ethylene and propylene containing from 20 to about 70 weight percent ethylene can be prepared using a catalyst system comprising trihexyl aluminum and vanadium oxychloride. From this technological base, processes for the production of the commercially important elastomeric ethylene-propylene copolymers, commonly referred to as EP elastomers, and the elastomeric terpolymers of ethylene, propylene and a nonconjugated diolefin, generally referred to as EPDM elastomers, have been developed.
In most current EP and EPDM production processes the catalysts used for the production of the high molecular weight elastomers are soluble catalysts formed from vanadium compounds such as vanadium tetrachloride, vanadium triacetylacetonate, vanadium trialkoxides and halogenated vanadium alkoxides which may be used singly or as a mixture in conjunction with an organoaluminum compound such as triethyl aluminum, diethyl aluminum chloride or ethyl aluminum sesquichloride.
With the recent advent of metallocene-alumoxane coordination catalyst systems for the production of polyethylene and copolymers of ethylene and alpha-olefins such as linear low density polyethylene, some effort has been directed to determining the suitability of particular metallocene-alumoxane catalyst systems for the production of EP and EPDM elastomers.
Kaminsky et al in J. Poly. Sci. Polymer Chem Edition, Volume 23, pp 2151-64 (1985) disclose the use of a soluble bis(cyclopentadienyl) zirconium dimethylalumoxane catalyst system in the polymerization of ethylene, propylene and 5-ethylidene-2-norbornene in toluene solution. By employing the catalyst at low zirconium concentrations, high aluminum to zirconium ratios and long reaction times, Kaminsky et al have been able to prepare, in low yields, high molecular weight EPDM elastomers with a high level of 5-ethylidene-2-norbornene incorporation. However, because of the long induction times required for the catalyst to reach its full activity, a period of the order of hours without the nonconjugated diolefin present, and longer with the nonconjugated diolefin present, precludes the use of such a catalyst system in a commercial operation as such long residence times are economically infeasible.
European patent application 347,129 (Floyd et al to Exxon) discloses that a catalyst system comprising an alkylene or silanylene or mixed alkylene-silanylene bridged bis(substituted cyclopentadienyl) Group IV B transition metal compound and an alumoxane is highly active in the production of high molecular weight EPDM elastomers that have a high ethylene content, a nonconjugated diolefin content of greater than about 3 weight percent and exhibit a low heat of fusion, that is have a low degree of crystallinity. However, in order for this high activity to be realized, the catalyst system has to be supported and a temperature of 50.degree. C. employed. Furthermore, the ethylene pressure used has to be higher than that typically used in the current EP and EPDM production processes which utilize soluble vanadium compounds as catalysts.
U.S. Pat. No. 5,001,205 (Hoel to Exxon) discloses that an elastomeric ethylene-alpha-olefin copolymer may be produced by a slurry polymerization process employing a highly active catalyst system comprising a bis(cyclopentadienyl) group IV B transition metal compound wherein each cyclopentadienyl moiety bears at least two substituents and the two cyclopentadienyl moieties may also be bridged with a linear, branched or cyclic alkylene group, an alkyl substituted silaalkylene group or an alkyl substituted silanylene group and an alumoxane. However, in order for the high activity to be obtained, the catalyst system is first supported on an inert support such as dehydrated silica gel and then is treated with a small amount of ethylene so that an amount of polymer is formed on the catalyst material. The catalyst system thus prepared is then utilized in the preparation of a high molecular weight elastomeric ethylene-alpha-olefin copolymer.
Heretobefore the use of a catalyst system comprising a metallocene wherein there are two transition metal centres and an alumoxane for the copolymerization of ethylene and an alpha-olefin or the terpolymerization of ethylene, an alpha-olefin and a nonconjugated diolefin has not been described.